Some optical instruments, such as telescopes and/or the like make use of large mirrors (e.g. 18″ and greater in diameter). Such large mirrors can be used at the primary (light gathering) mirrors of such optical instruments. Mirrors for optical instruments are typically formed by coating a reflective material (such as silver) onto a surface of a mirror blank typically fabricated from glass.
Traditionally, to provide sufficient structural rigidity, large mirror blanks (e.g. 18″ and greater in diameter) are made of a single solid piece of glass. The solid piece of glass is preheated until soft and then forged into the desired shape. After forging, the glass is cooled and drafted.
After cooling and drafting, there typically exists a large amount of residual stress within the glass that forms the blank. To reduce residual stress, the glass that forms the blank is typically annealed. Annealing involves controllably and slowly heating the glass to an annealing temperature, heat soaking the glass at the annealing temperature and then slowly and controllably decreasing the temperature of the glass. Annealing takes considerable time (e.g. hundreds of hours). It is not uncommon for the glass to crack during the heating and cooling processes associated with annealing.
To prevent optical distortion which may be caused, for example, by deformation of a solid glass mirror under its own weight, telescope frames have to be strong and rigid. This requirement raises the cost of manufacturing telescopes and leads to telescopes that are more difficult to assemble, disassemble and transport.
There is a general desire for improved mirror blanks and improved methods to manufacture mirror blanks.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.